1. Field of the Invention
The present invention relates generally to the manufacture of plastic pipe of the type used in municipal water and sewer systems, more specifically, to the injection and compression molding processes used to form the sealing gaskets which are installed in such systems, and to a process for recovering and further utilizing waste energy from such processes.
2. Description of the Prior Art
An almost unlimited variety of articles are produced from rubber, synthetic elastomers and plastics using compression and injection molding equipment. As one example, many kinds of equipment, ducting, piping, castings, and other parts are joined together in gasketed joints in order to form gas, liquid, electrical, chemical, or sound isolation seals at the joints.
For example, a variety of piping systems are known for the conveyance of fluids which employ elastomeric type sealing rings or gaskets. The pipes used in such systems may be formed of PVC, polyolefins such as PE and PP, ductile iron, concrete, clay, fiberglass, steel, cast iron, fiberglass/cement reinforced pipes and such metals as aluminum and copper. Pipes formed from thermoplastic materials including polyethylene and PVC are used in a variety of industries but are particularly useful in municipal water and sewage systems. It is well known in the art to extrude such plastic pipes in an elongated cylindrical configuration of a desired diameter and to then cut the extruded product into individual lengths of convenient size suitable for handling, shipping and installing. Each length of pipe is enlarged or “belled” at one end sufficiently to join the next adjacent pipe section by receiving in the female, belled end the unenlarged or “spigot” male end of the next adjacent length of pipe. The inside diameter of the bell is formed sufficiently large to receive the spigot end of the next section of pipe with sufficient clearance to allow the application of packing, caulking, elastomeric gaskets or other sealing devices designed to prevent leakage at pipe joints when a plurality of pipe lengths are joined to form a pipeline.
In the early 1970's, a new sealing gasket technology for PVC pipes was developed by Rieber & Son of Bergen, Norway, referred to in the industry as the “Rieber Joint.” The Rieber system provided an integral sealing mechanism within the belled or female pipe end for sealing with the spigot end of a mating pipe formed from thermoplastic material. In the Rieber process, the elastomeric gasket was inserted within an internal groove in the socket end of the female pipe as the female or belled end was simultaneously being formed. The provision of a prestressed and anchored elastomeric gasket during the belling process at the pipe factory provided an improved socket end for a pipe joint with a sealing gasket which would not twist or flip or otherwise allow impurities to enter the sealing zones of the joint, thus increasing the reliability of the joint and decreasing the risk of leaks or possible failure due to abrasion. The Rieber process is described in the following issued United States patents, among others: U.S. Pat. Nos. 4,120,521; 4,061,459; 4,030,872; 3,965,715; 3,929,958; 3,887,992; 3,884,612; and 3,776,682.
The elastomeric rings or gaskets used in the above type of sealing applications are typically formed from natural or synthetic rubbers or synthetic elastomers in compression or injection molding machines. Either type of molding machine produces excess, waste heat in the manufacturing environment. Heat is also utilized in the Rieber pipe manufacturing process where the female pipe end is heated and forced over the forming mandrel and sealing gasket to form the belled pipe end. Despite the many advances which have occurred in injection and compression molding technologies, and in plastic pipe manufacturing techniques, a need exists to continue to improve the manufacturing efficiency and economy in such processes. A specific area in which the overall efficiency and economy of the processes can be improved is in the area of energy utilization and recovery.
Both profits and environmental quality can be improved by more carefully using energy in industrial processes such as the compression and injection molding processes and plastic pipe manufacturing processes described. Discharging any heated fluid (air, water, etc.) into the environment is a waste of money and resources. Hot waste streams are paid for with money that could have been profit. Energy has always been a significant component of industrial operations, but only rarely has efficient energy use been a priority when factories were being expanded. As long as a factory or product line was profitable, very little attention was usually paid to efficient energy use. Today, however, increasing attention is being focused on maximizing energy use and minimizing pollution in the modern industrial setting.